1. Field of the Invention
This invention relates to a semiconductor device and more particularly to a semiconductor device having a photo-transistor function.
2. Description of the Prior Art
The conventional photo FET (photo Field Effect Transistor) functions as a photo diode. FIG. 1 is a circuit representation of the conventional photo FET. A gate of the photo FET 1 is connected to a gate control circuit comprising a power source Vo, indicated by reference numeral 32, through a resistor R.sub.g. A resistor R and a power source V are connected between a source of the photo FET 1 and a drain thereof. The drain of the photo FET 1 is connected also to the power source V.sub.o. A photocurrent I.sub.e flows through the resistor R.sub.g with the irradiation of light having energy h.nu.. An effective voltage V.sub.g applied to the gate is represented by V.sub.g = V.sub.o -R.sub.g xI.sub.e. The change of the voltage V.sub.g is detected by that of the voltage of the gate region of the photo FET 1. A relationship R.sub.g .apprxeq. V.sub.o /I.sub.e is required in order that the gate voltage V.sub.g changes greatly with the photocurrent I.sub.e.
Since the photo FET 1 is of a junction type, a diode is formed between the gage region of the photo FET 1 and the semiconductive substrate thereof. The diode functions as the photo diode. Accordingly, the photo FET 1 has the same characteristic as the normal photo diode (FIG. 2). When no light is incident on the photo FET 1, the gate current I.sub.g is zero. When light with the intensity 500 1.times. of illumination L and light with the intensity 1000 1.times. of illumination L are incident on the FET 1 from an incandescent electric lamp, the gate currents I.sub.g vary with the gate voltages V.sub.g, as shown by curves a and b. For the 1000 1.times. light, R.sub.g .apprxeq. 5 M.OMEGA. is obtained from the curve a. For the light with the lower intensity of illumination, the gate resistance R.sub.g is required to be larger. Accordingly, it is troublesome to design the circuit, particularly in an integrated form.
The channel resistance of the photo FET 1 can be changed by another method in which the gate of the photo FET 1 is opened and the photo FET 1 is self-biased by photo electromotive force. However, the effective gate voltage is equal to the open-circuit voltage of the normal photo diode. Therefore, the gate voltage cannot be changed beyond 1V by the irradiation of light. For a great change of the resistance, a junction type FET should be used the channel of which is normally pinched off. It is difficult to manufacture such a junction type FET.